An aerial photo of a neighborhood hit by a massive tornado in Moore, on May 20, 2013. (AP Photo/Steve Gooch)

An aerial photo of homes hit by the tornado in Moore, Oklahoma, on May 20, 2013. (AP Photo/Steve Gooch)

An aerial photo showing damage to Plaza Towers Elementary School after it was hit by a massive tornado in Moore, on May 20, 2013. Rescue workers and a helicopter can be seen at lower right. (AP Photo/Steve Gooch)

Rescue workers dig through the rubble of a collapsed wall at the Plaza Tower Elementary School to free trapped students in Moore, on May 20, 2013. (AP Photo/Sue Ogrocki)

A boy is pulled from beneath a collapsed wall at the Plaza Towers Elementary School, on May 20, 2013. (AP Photo/ Sue Ogrocki)

A woman walks through what remains of a bowling alley, and other buildings, after a huge tornado struck Moore, near Oklahoma City, on May 20, 2013. (Reuters/Richard Rowe)

An aerial photo shows the remains of tornado-damaged houses in Moore, on May 20, 2013. A tornado roared through the Oklahoma City suburbs Monday, flattening entire neighborhoods, setting buildings on fire and landing a direct blow on an elementary school. (AP Photo/Steve Gooch)